Conventionally, there are known photoreactive polysaccharides obtained by introducing a photoreactive group into polysaccharides, as well as crosslinked polysaccharides obtained by crosslinking the photoreactive polysaccharides by irradiation of light such as ultraviolet rays (for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 6-73102 (1994), 8-143604 (1996), 9-87236 (1997) and 2002-249501). In addition, there are also known gels (polysaccharide gels) or sponges (polysaccharide sponges) produced from such crosslinked polysaccharides.
The polysaccharide gels have been produced by irradiating a solution of the photoreactive polysaccharide with light such as ultraviolet rays for crosslinking the photoreactive polysaccharide, and used, for example, as medical materials such as antiadhesive materials for inhibiting adhesion of tissues of living organisms (for example, Japanese Patent Application Laid-Open (TOKUHYO) No. 11-512778 (1999)). Meanwhile, the above polysaccharide gels are obtained in the form of a solvated gel. When using an aqueous solution of photoreactive polysaccharides as a raw material, the resultant polysaccharide gels are in the form of a hydrogel due to hydration thereof. Such polysaccharide gels have a three-dimensional network structure, and therefore, are insoluble in water, but swelled up until reaching an equilibrium condition thereof in water.
On the other hand, the polysaccharide sponges are produced by freezing a solution of photoreactive polysaccharides and then irradiating the frozen solution with light such as ultraviolet rays to crosslink the photoreactive polysaccharides. In the production process, impurities such as crosslinking agents are extremely easily removed from the reaction mixture, thereby enabling production of high-purity products (for example, WO 02/060971 A1). Meanwhile, the term “sponge” means a porous substance having closed cells or interconnecting cells.